| 1. |
- Slapak, Rikard, et al.
(författare)
-
A statistical study on O+ flux in the dayside magnetosheath
- 2013
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31, s. 1005-1010
-
Tidskriftsartikel (refereegranskat)abstract
- Studies on terrestrial oxygen ion (O+) escape into the interplanetary space have considered a number of different escape paths. Recent observations however suggest a yet insufficiently investigated additional escape route for hot O+: along open magnetic field lines in the high altitude cusp and mantle. Here we present a statistical study on O+ flux in the high-latitude dayside magnetosheath. The O+ is generally seen relatively close to the magnetopause, consistent with observations of O+ flowing primarily tangentially to the magnetopause. We estimate the total escape flux in this region to be ~ 7 × 1024 s−1, implying this escape route to significantly contribute to the overall total O+ loss into interplanetary space.
|
|
| 2. |
- Kirkwood, Sheila, et al.
(författare)
-
Response of polar mesosphere summer echoes to geomagnetic disturbances in the Southern and Northern Hemispheres : The importance of nitric oxide
- 2013
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31:2, s. 333-347
-
Tidskriftsartikel (refereegranskat)abstract
- The relationship between polar mesosphere summer echoes (PMSE) and geomagnetic disturbances (represented by magnetic I K indices) is examined. Calibrated PMSE reflectivities for the period May 2006-February 2012 are used from two 52.0/54.5 MHz radars located in Arctic Sweden (68 N, geomagnetic latitude 65 ) and at two different sites in Queen Maud Land, Antarctica (73/72 S, geomagnetic latitudes 62/63 ). In both the Northern Hemisphere (NH) and the Southern Hemisphere (SH) there is a strong increase in mean PMSE reflectivity between quiet and disturbed geomagnetic conditions. Mean volume reflectivities are slightly lower at the SH locations compared to the NH, but the position of the peak in the lognormal distribution of PMSE reflectivities is close to the same at both NH and SH locations, and varies only slightly with magnetic disturbance level. Differences between the sites, and between geomagnetic disturbance levels, are primarily due to differences in the high-reflectivity tail of the distribution. PMSE occurrence rates are essentially the same at both NH and SH locations during most of the PMSE season when a sufficiently low detection threshold is used so that the peak in the lognormal distribution is included. When the local-time dependence of the PMSE response to geomagnetic disturbance level is considered, the response in the NH is found to be immediate at most local times, but delayed by several hours in the afternoon sector and absent in the early evening. At the SH sites, at lower magnetic latitude, there is a delayed response (by several hours) at almost all local times. At the NH (auroral zone) site, the dependence on magnetic disturbance is highest during evening-to-morning hours. At the SH (sub-auroral) sites the response to magnetic disturbance is weaker but persists throughout the day. While the immediate response to magnetic activity can be qualitatively explained by changes in electron density resulting from energetic particle precipitation, the delayed response can largely be explained by changes in nitric oxide concentrations. Observations of nitric oxide concentration at PMSE heights by the Odin satellite support this hypothesis. Sensitivity to geomagnetic disturbances, including nitric oxide produced during these disturbances, can explain previously reported differences between sites in the auroral zone and those at higher or lower magnetic latitudes. The several-day lifetime of nitric oxide can also explain earlier reported discrepancies between high correlations for average conditions (year-by-year PMSE reflectivities and indices) and low correlations for minute-to-day timescales
|
|
| 3. |
- Mihalikova, Maria, et al.
(författare)
-
Tropopause fold occurrence rates over the Antarctic station Troll (72 degrees S, 2.5 degrees E)
- 2013
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31:4, s. 591-598
-
Tidskriftsartikel (refereegranskat)abstract
- One of the important mechanisms of stratosphere-troposphere exchange, which brings ozone-rich stratospheric air to low altitudes in extratropical regions, is transport related to tropopause folds. The climatology of folds has been studied at high latitudes of the Northern Hemisphere with the help of radars and global models. Global models supply information about fold occurrence rates at high latitudes of the Southern Hemisphere as well, but so far comparisons with direct measurements are rare. The Moveable Atmospheric Radar for Antarctica (MARA), a 54.5 MHz wind-profiler radar, has been operated at the Norwegian year-round station Troll, Antarctica (72 degrees S, 2.5 degrees E) since December 2011. Frequent tropopause fold signatures have been observed. In this study, based on MARA observations, an occurrence rate statistics of tropopause folds from December 2011 until November 2012 has been made, and radar data have been compared with the analysis from the European Center for Medium-Range Weather Forecasting (ECMWF). The fold occurrence rates exhibit an annual cycle with winter maximum and summer minimum and suggest significantly higher occurrence rates for the given location than those obtained previously by global model studies.
|
|
| 4. |
- Christensen, Sören, 1982
(författare)
-
Optimal decision under ambiguity for diffusion processes
- 2013
-
Ingår i: Mathematical Methods of Operations Research. - : Springer Science and Business Media LLC. - 1432-2994 .- 1432-5217. ; 77:2, s. 207-226
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we consider stochastic optimization problems for an ambiguity averse decision maker who is uncertain about the parameters of the underlying process. In a first part we consider problems of optimal stopping under drift ambiguity for one-dimensional diffusion processes. Analogously to the case of ordinary optimal stopping problems for one-dimensional Brownian motions we reduce the problem to the geometric problem of finding the smallest majorant of the reward function in a two-parameter function space. In a second part we solve optimal stopping problems when the underlying process may crash down. These problems are reduced to one optimal stopping problem and one Dynkin game. Examples are discussed. © 2013 Springer-Verlag Berlin Heidelberg.
|
|
| 5. |
- Aikio, Anita T., et al.
(författare)
-
IMF effect on the polar cap contraction and expansion during a period of substorms
- 2013
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31, s. 1021-1034
-
Tidskriftsartikel (refereegranskat)abstract
- The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase.In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.
|
|
| 6. |
- Arhammar, C., et al.
(författare)
-
A theoretical study of possible point defects incorporated into alpha-alumina deposited by chemical vapor deposition
- 2013
-
Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 133:2, s. 1433-
-
Tidskriftsartikel (refereegranskat)abstract
- The energetics and electronic structure of carbon, chlorine, hydrogen, and sulfur in alpha-Al2O3 was investigated by first principles and thermodynamical calculations. These species are present in the gas phase during the synthesis of alpha-Al2O3 by chemical vapor deposition (CVD) but little is known of their solubility in this compound. The heat of formation from standard reference states of the elements varying the chemical potential of each element was calculated. An attempt to model the actual conditions in the CVD process was made, using the species and solid compounds present in a common CVD process as reference states. Our calculations suggest that sulfur from the catalyzing agent H2S will not solve in alpha-Al2O3 during deposition by CVD. It is found that the neutral chlorine and hydrogen interstitial defects display the lowest heat of formation, 281 and 280 kJ/mol, respectively, at the modeled CVD conditions. This energy is too high in order for neutral defects to form during CVD of alpha-Al2O3 at any significant amounts. The charged defects and their compensation were studied. Carbon substituting oxygen is found to be energetically favored under the modeled CVD conditions, considering carbon dioxide as competing species to solid solubility in alpha-Al2O3 at an energy of -128 kJ/mol. However, care needs to be taken when choosing the possible competing carbon-containing phases. Compensation of carbon substituting for oxygen by oxygen vacancies takes place at 110 kJ/mol from standard reference states, graphite, fcc-Al and O-2. The carbon solubility in Al2O3 is difficult to measure with standard analysis techniques such as X-ray diffraction and energy dispersive X-ray spectroscopy, but several stable compounds in the Al-C-O are available in the literature.
|
|
| 7. |
- Barata-Morgado, Rute, et al.
(författare)
-
Theoretical study of the conformational equilibrium of 1,4-dioxane in gas phase, neat liquid, and dilute aqueous solutions
- 2013
-
Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 132:10, s. 1390-
-
Tidskriftsartikel (refereegranskat)abstract
- The conformational equilibrium of 1,4-dioxane in the gas phase, in the pure liquid, and in aqueous solution has been studied by means of the Average Solvent Electrostatic Potential from Molecular Dynamics (ASEP/MD) method and the Integral Equation Formalism for the Polarizable Continuum Model (IEF-PCM). The dioxane molecule was described at the DFT(B3LYP)/aug-cc-pVTZ level. In the three phases, the equilibrium is almost completely shifted toward the chair conformer, with populations of the twist-boat conformers lower than 0.01 %. The equilibrium is dominated by the internal energy of the molecule, as the solute-solvent interaction free energies are very similar in the three conformers considered (chair, 1,4 twist-boat, and 2,5 twist-boat). In the pure liquid, where the dioxane-dioxane interaction is dominated by the Lennard-Jones term, the structure is characteristic of a van der Waals liquid. However, the decrease in the C-H distance from gas phase to solution, the increase in the C-H vibrational frequencies, and the presence of a shoulder in the O-Haxial pair radial distribution function point to the presence of a weak C-H-O hydrogen bond. The analysis of the occupancy maps of water oxygen and hydrogen atoms around the 1,4-dioxane molecule confirms this conclusion. Contrary to what is found in small water-dioxane clusters, in the liquid, there is a preference for oxygen atoms to interact with axial hydrogen atoms to form C-H-O hydrogen bonds. Comparison of ASEP/MD and IEF-PCM results indicates that including specific interactions is very important for an adequate description of the solute-solvent interaction; however, the influence of these interactions does not translate in changes in the relative stability of the conformers because it cancels out when energy differences are calculated.
|
|
| 8. |
- Danielsson, Örjan, 1973-, et al.
(författare)
-
Shortcomings of CVD modeling of SiC today
- 2013
-
Ingår i: Theoretical Chemistry accounts. - : Springer Berlin/Heidelberg. - 1432-881X .- 1432-2234. ; 132:11, s. 1398-
-
Tidskriftsartikel (refereegranskat)abstract
- The active, epitaxial layers of silicon carbide (SiC) devices are grown by chemical vapor deposition (CVD), at temperatures above 1,600 °C, using silane and light hydrocarbons as precursors, diluted in hydrogen. A better understanding of the epitaxial growth process of SiC by CVD is crucial to improve CVD tools and optimize growth conditions. Through computational fluid dynamic (CFD) simulations, the process may be studied in great detail, giving insight to both flow characteristics, temperature gradients and distributions, and gas mixture composition and species concentrations throughout the whole CVD reactor. In this paper, some of the important parts where improvements are very much needed for accurate CFD simulations of the SiC CVD process to be accomplished are pointed out. First, the thermochemical properties of 30 species that are thought to be part of the gas-phase chemistry in the SiC CVD process are calculated by means of quantum-chemical computations based on ab initio theory and density functional theory. It is shown that completely different results are obtained in the CFD simulations, depending on which data are used for some molecules, and that this may lead to erroneous conclusions of the importance of certain species. Second, three different models for the gas-phase chemistry are compared, using three different hydrocarbon precursors. It is shown that the predicted gas-phase composition varies largely, depending on which model is used. Third, the surface reactions leading to the actual deposition are discussed. We suggest that hydrocarbon molecules in fact have a much higher surface reactivity with the SiC surface than previously accepted values.
|
|
| 9. |
- Jacob, Stefan, et al.
(författare)
-
Noise-induced alterations in cochlear mechanics, electromotility, and cochlear amplification
- 2013
-
Ingår i: Pflügers Archiv. - : Springer. - 0031-6768 .- 1432-2013. ; 465:6, s. 907-917
-
Tidskriftsartikel (refereegranskat)abstract
- Loud sounds are a common cause of hearing loss. Very intense sounds may result in permanent hearing loss, but lower levels typically cause a transient decrease in auditory sensitivity. Studies have arrived at different conclusions as regards the physiological mechanisms underlying such temporary threshold shifts. Here, we investigated the effect of acoustic overstimulation on the mechanics of the low-frequency areas of the guinea pig cochlea. We demonstrate that brief loud sound exposure results in an increased phase lag and a paradoxical frequency-specific increase of sound-evoked displacement. Despite the increased displacement, electrically evoked motion is reduced. Because electromotility is important for amplifying low-level sounds, this change was associated with a decrease in measures of cochlear amplification. These changes recovered over the course of 30-40 min. Overstimulation also caused an increase in cytoplasmic calcium levels of both hair cells and supporting cells. These data suggest that reduced organ of Corti stiffness contributes to temporary threshold shifts.
|
|
| 10. |
- Kalered, Emil, et al.
(författare)
-
Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations
- 2013
-
Ingår i: Theoretical Chemistry accounts. - : Springer Verlag (Germany). - 1432-881X .- 1432-2234. ; 132:12
-
Tidskriftsartikel (refereegranskat)abstract
- The effect chlorine addition to the gas mixture has on the surface chemistry in the chemical vapour deposition (CVD) process for silicon carbide (SiC) epitaxial layers is studied by quantum-chemical calculations of the adsorption and diffusion of SiH2 and SiCl2 on the (000-1) 4H–SiC surface. SiH2 was found to bind more strongly to the surface than SiCl2 by approximately 100 kJ mol−1 and to have a 50 kJ mol−1 lower energy barrier for diffusion on the fully hydrogen-terminated surface. On a bare SiC surface, without hydrogen termination, the SiCl2 molecule has a somewhat lower energy barrier for diffusion. SiCl2 is found to require a higher activation energy for desorption once chemisorbed, compared to the SiH2 molecule. Gibbs free energy calculations also indicate that the SiC surface may not be fully hydrogen terminated at CVD conditions since missing neighbouring pair of surface hydrogens is found to be a likely type of defect on a hydrogen-terminated SiC surface.
|
|
